Fresh fighting between the terrorist group of the Islamic State of Iraq and the Levant (ISIL), a coalition of militants and an Al-Qaeda affiliate in eastern Syria has left at least 45 fighters dead, an activist group says.

ISIL, which grew from a former affiliate of Al-Qaeda in Iraq, has faced a backlash in opposition Syria since early this year and is battling a coalition including Al-Qaeda's Nusra Front and other militant groups.

The so-called Syrian Observatory for Human Rights said the clashes in the town of Khosham in the eastern province of Deir Al-Zor left 17 Nusra and extremist fighters dead, along with 28 ISIL members.

The fighting began Sunday and raged through the night, the Observatory said.

The fight against ISIL has killed some 6,000 people since it began in early January, according to the Observatory, a Britain-based organization.